Heating Help

MERV Rating

I understood that MERV was measured from 1-16. While explaining the filter ratings to a customer, I was interrupted by someone and was told that MERV goes as high as 21.

I've done a little research, and I've found 16 on most websites, but some are showing 20, and one even mentioned it can be infinite.

Can you clear the air for me, Bill?

MERV numbers

MERV is the abbreviation for minimum efficiency reporting value of how many particles a particular filter removes. They range from 1 to 16, with the disposable "home center" 1"panels at merv 1-4. Pleated media filters both retail and professional range from 4 to 12, with HEPA (high energy particle arrestance) filters at the top level of 16.  HEPA's are tested using a chemical that produces uniform size particles that are counted both upstream and downstream from the filter.  HEPAS remove about 99% of .3 micron particles.  HEPA-type and HEPA-like are NOT true HEPA filters!  If you are designing a system, remember that the filter will gain resistance as it loads, and especially if you are using HEPAS, size your fans carefully.  Electronic air cleaners don't have a MERV rating. I am not aware of anything higher than MERV16, other than a sheet of 1/2 inch plywood!